Affected versions

• 83.0b5 (20201027175448)
• 84.0a1 (20201027212052)

Affected platforms

• macOS 10.12

Steps to reproduce
The hang happens very often but unfortunately, I don't have some reliable steps in order to reproduce this issue. This happens when doing random stuff like: open random web pages in new tabs, closing firefox with multiple tabs opened, restarting firefox multiple times, sign in to sync, surfing the web for a long period of time, turn on/off tracking protection, opening from profile manager.

Regression range

• I cannot reproduce the issue with Firefox 83.0b4 (20201025174155), I tried multiple times and firefox works fine. I can’t tell for sure if this is not affected because the issue is intermittent and happens randomly.

Notes

• No crash logs are recorded in about:crashes after force quitting firefox.
• I am attaching here the Apple reporter after force quitting Firefox.
• From what I saw, leaving firefox open with some pages leads to a hang and the CPU is 100%. See screenshots here and here.
• About:support content here
• Screenshot with 100% process while reloading cnn page: link
• Screen recording: link

Suggested Severity: S2

I just got this crash report after closing firefox: https://crash-stats.mozilla.org/report/index/da001515-3e41-429b-82dc-47a930201028#tab-details. I don't know if these hangs are related.

Could be caused by bug 1665411, NI Gabriele.

Yeah, it seems that condition variables don't play nice w/ first-fit mutexes in macOS versions older than 10.14. We have to back out bug 1665411 and then I'll rework the patch.

Fixed by backout.